1. Field of the Invention
This invention relates generally to electrical signal transmission, and more particularly to shielded transmission cable having a high degree of flexing endurance, compactness, and improved mechanical and electrical characteristics.
2. Description of Prior Art
Twin-axial cable is generally formed by encasing two electric conductors, separated a predetermined distance, within a dielectric layer. The distance of separation between the two conductors is selected, in part, to prevent degradation of the electric signal. Signal degradation occurs when there is a change in the electric signal. The electrical characteristics which represent two forms of degradation are the "attenuation" ratio and the "characteristic impedance of transmission" value. Both of these characteristics can be partially controlled by the distance of separation between the two conductors. It is desirable, therefore, to maintain a selected distance of separation between the conductors since the distance can affect these two characteristics, which in turn affects the signal degradation and overall performance of the cable.
A significant problem in connection with the distance of separation between the two conductors is in maintaining the separation along the entire length of the cable during manufacturing. Twin conductor cables made in accordance with the prior art require a high degree of skill to ensure a desired distance of separation between the two conductors. Although a cable design has specific requirements, meeting the requirements is often heavily dependent on the skill level of the particular manufacturer. Another problem occurs in adjusting the distance of separation between the conductors to meet specific design needs. Typically, in prior art, significant alterations to a cable design must be made to effectuate new electrical requirements. These alterations, in turn, result in significant changes in the manufacturing process. It is desirable when addressing conductor separation problems, therefore, to decrease the skill level requirements of the manufacturer and design a cable that is easily altered to meet specific electrical needs.
The dielectric layer that encases the conductors serves several purposes. First, it is used to physically hold the conductors in their proper geometric configuration. Second, it serves as a protective barrier for the conductors. In certain applications, a cable may be subject to harsh chemical and physical environments, such as extreme temperatures, high pressures, and corrosive conditions. If the conductors come in contact with this environment, signal degradation or physical damage can occur.
A common problem with the dielectric layer arises in its capacity as a protective barrier. If the layer is not sufficiently thick around the conductors, cracks that form in the dielectric layer can penetrate to the conductors, exposing them to the outside environments. However, if the layer is too thick, the cable is less flexible. Another problem which stems from encasing the conductors with the dielectric layer is the formation of interstices. If the encasing process is not properly performed, these interstices often form between the conductors and the dielectric layer. Formation of the interstices compounds the problems associated with the formation of cracks in the dielectric layer, by allowing penetration of undesirable materials into the critical regions to maintain signal integrity.
FIGS. 1 and 2 depict on type of twin-axial cable 36 of the prior art. This cable consists of two insulated conductors 30, 30', twisted together, and encased in a dielectric layer 31. In forming the cable 36, the first step involves surrounding each conductor 30, 30' with a large diameter insulation jacket 32, 32' The thickness of the jacket is selected to be one-half the desired distance of separation between the two conductors. In the next step, the insulated conductors are twisted together. When this step is complete, the conductors are properly separated because of the thickness of the insulation jacket around each conductor. A dielectric layer 31 is then formed around the twisted, insulated conductors. Next, a shield 33 is braided over the dielectric layer. Finally, an outside jacket 35 is formed around the braided shield 33.
There are several disadvantages with the twin-axial cable illustrated in FIGS. 1 and 2. The dielectric 31 tends to crack at point "a", where the thickness of the dielectric is relatively small compared to the thickness at point "b". The dielectric layer 31 in this example compounds the problem because of the thin sections closest to the insulated conductors 30, 30'. Further, if the insulated conductors 30, 30' do not remain substantially concentric with the dielectric layer 31 during manufacturing, portions of the insulated conductors in the areas where the layer thickness is small, i.e. point "a", may not be covered at all by the layer. Depending on the skill of the manufacturer, therefore, these thin sections of the dielectric layer 31 provide, at best, only a comparatively thin barrier of protection for the insulated conductors, and at worst, provide no barrier of protection at all.
A second disadvantage with the cable of FIGS. 1 and 2 also stems from the thick and thin sections of the dielectric layer 31 encasing the insulated conductors 30, 30'. Typically, the dielectric layer is encased around the insulated conductors by extrusion. The extrusion process ideally produces a cylindrical extrusion that is substantially concentric with the twisted, insulated conductors. The physical design of the cable 36, however, creates difficulties in achieving this goal because of material shrinkage of the dielectric layer 31 upon cooling of the thick and thin sections. Material shrinkage depends on the material. Since the thickness of the dielectric layer material is not the same around the cross-section, the shrinkage of sections of varying thickness is not uniform. Thicker sections such as that of point "b" will tend to shrink more than the thinner section of point "a". The disparity in shrinkage provides a cross section of the extrusion that is not substantially cylindrical. The non-cylindrical shape of the extrusion results in additional problems and expense in making and installing the braided shield 33.
The design of the cable of FIGS. 1 and 2 may also result in the formation of interstices. The materials selected for the dielectric layer 31 and the insulation jackets 32, 32' generally do not have the same chemical base. When the dielectric layer is extruded around the insulated conductors, the two materials may not bond due to the difference in the two materials. At the points where a bond does not form between the two materials, interstices form instead.
FIGS. 3 and 4 depict a second method of making a twin-axial cable. A cable 26 consists of two uninsulated conductors 20, 20', separated by a predetermined distance, and encased in a dielectric layer 21. In forming the cable 26, the first step involves extruding a dielectric layer 21 around the two uninsulated conductors 20, 20'. As the dielectric is extruded around the conductors, the conductors are held substantially parallel to each other at the required distance. Again, a braided shield 23 is placed around the extrusion. Finally, a protective jacket 25 is extruded over the braid.
This type of twin-axial configuration has an advantage over the twin-axial cable of FIGS. 1 and 2 in that there is no opportunity for interstices to form around the conductors since there are no conductor insulation jackets to which the dielectric layer material must bond. However, the advantages of this method over the previously described method are outweighed by the loss of flexibility of the cable. As shown in FIGS. 3 and 4, the conductors are fixed in the same geometric plane for the entire length of the cable. When the cable is flexed, or bent, orthogonally to the plane containing the conductors, each conductor has an equal amount of tension and compression placed upon it. However, if the cable is made to flex so that it is bent in the plane of the conductors, so that the direction of the bend is coplanar with the two conductors, one of the conductors will be placed in a much higher state of tension than the other conductor. This results in a cable which is stiff and more susceptible to fatigues stress.
The present invention provides a cable which has improved mechanical and electrical characteristics over prior art.